An alkyne A ( $C X_{6} H X_{10}$ ) gives only propanoic acid on ozonolysis followed by aqueous work-up. A is reduced with Na in liquid ammonia to give B, and B is treated with $Br X_{2}$ in $CCl X_{4}$ to give C. C is

Answer: C

Only propanoic acid means A is symmetrical, hex-3-yne,

CH X_{3} CH X_{2} - C \equiv C - CH X_{2} CH X_{3}

. Na/liquid

NH X_{3}

gives anti addition of

H X_{2}

, so B is (E)-hex-3-ene. Bromine adds anti to the trans-alkene, giving the meso dibromide, meso-(3R,4S)-3,4-dibromohexane.